Emulsified water contamination of fuel, particularly diesel fuel, jet fuel, biodiesel fuel, ethanol, butanol, or a blend thereof, is a serious concern for the operation of fossil fuel powered engines. Water can become stably entrained in such fuels, especially in diesel fuel formulations containing high concentrations of additives where the presence of water and waterbased impurities can cause fuel filter plugging, fuel starvation, damage of engine components by cavitation or corrosion, promotion of microbiological growth, and other problems.
Separation of contaminant water from fuel has posed a particularly challenging problem. Industrial water-in-oil emulsions can be separated by gravity, centrifugation, coalescence, absorption, distillation, and headspace dehumidification. Simple mechanical devices based on separation by gravity or mild centrifugal force are satisfactory if the free water is present as a discrete second phase. However, free water is often emulsified by pumps and valves, and may remain as a stable emulsion, especially in diesel or jet fuel saturated with surfactants. Two-stage coalescer/separators are designed to remove water emulsions. The coalescer breaks the emulsion by preferential wetting of fibrous materials such as fiber glass. The water is accumulated into large droplets and is removed by gravity separation against a hydrophobic separator material such as Teflon coated wire cloth or silicone impregnated paper. The presence of wetting agents or surfactants may interfere with the coalescence of water emulsions, especially in jet and diesel fuels.
The conventional mechanism of coalescence in a fibrous bed is explained by the Hazlett model. The model consists of four main steps: 1) approach of a droplet to a fiber, 2) attachment of the droplet to the fiber, 3) coalescence of attached droplets on the fiber, and 4) release of enlarged droplets from the downstream side of the fiber bed. Thus, coalescing water droplets from the hydrocarbon phase requires a hydrophilic site in a fibrous bed in order to attach water droplets to the fibers. The glass fiber medium applied in conventional coalescer devices has a surface which has both hydrophilic (e.g. silanol, cellulose) and hydrophobic (silicone, organic resin) regions. It is widely accepted that water interception and growth occurs at the hydrophilic sites. When surfactants are present in the fuel, the polar head of the surfactant can be adsorbed at the hydrophilic sites, causing failure of the coalescer unit. Failure can occur in two ways: the surfactants reduce interfacial tension and form a stable emulsion to prevent coalescence; or surfactants are adsorbed or coated on the fiber media, thereby changing characteristics such as wettability with regard to water, fuel, or both. This coating phenomenon has been evidenced by scanning electron microscopy (SEM). See Hughes, V. B., “Aviation Fuel Handling: New Mechanistic Insight Into The Effect Of Surfactants On Water-Coalescer Performance,” 2nd International Filtration Conference, San Antonio, U.S., Apr. 1-2, 1998, pp. 92-104.
This invention is concerned with coalescence as a means to separate water from emulsions in fuel. In particular, the invention utilizes specialized nonwoven fine fiber webs to capture and remove water from fuel emulsions, even very stable emulsions. Further, the fine fiber webs of the invention can separate waterborne impurities, such as hard or soft particulates such as dust, aggregated organic matter, or even bacteria from fuels when the impurities are entrained in a water phase within a fuel.
Non-woven webs for many end uses, including filtration media, have been manufactured for many years. Such structures are disclosed in, for example, Wincklhofer et al., U.S. Pat. No. 3,616,160; Sanders, U.S. Pat. No. 3,639,195; Perrotta, U.S. Pat. No. 4,210,540; Gessner, U.S. Pat. No. 5,108,827; Nielsen et al., U.S. Pat. No. 5,167,764; Nielsen et al., U.S. Pat. No. 5,167,765; Powers et al., U.S. Pat. No. 5,580,459; Berger, U.S. Pat. No. 5,620,641; Hollingsworth et al., U.S. Pat. No. 6,146,436; Berger, U.S. Pat. No. 6,174,603; Dong, U.S. Pat. No. 6,251,224; Amsler, U.S. Pat. No. 6,267,252; Sorvari et al., U.S. Pat. No. 6,355,079; Hunter, U.S. Pat. No. 6,419,721; Cox et al., U.S. Pat. No. 6,419,839; Stokes et al., U.S. Pat. No. 6,528,439; Amsler, U.S. Pat. No. H2,086, U.S. Pat. No. 5,853,439; U.S. Pat. No. 6,171,355; U.S. Pat. No. 6,355,076; U.S. Pat. No. 6,143,049; U.S. Pat. No. 6,187,073; U.S. Pat. No. 6,290,739; U.S. Pat. No. 6,540,801; and U.S. Pat. No. 6,530,969; Chung et al., U.S. Pat. No. 6,743,273; Chung et al., U.S. Pat. No. 6,924,028; Chung et al., U.S. Pat. No. 6,955,775; Chung et al., U.S. Pat. No. 7,070,640; Chung et al., U.S. Pat. No. 7,090,715; and Chung et al., U.S. Patent Publication No. 2003/0106294. Such structures have been applied and made by both air laid and wet laid processing and have been used in fluid, both gaseous and air and aqueous and non-aqueous liquid filtration applications, with some degree of success.
Many filter materials made from nonwoven webs have been directed to separating water contamination from fuel streams, e.g. hydrocarbons that are liquids at ambient temperatures. These filter materials may take advantage of the Hazlett model for the mechanism of coalescence. For example, Reiman, U.S. Pat. No. 3,142,612, describes a fuel filter for separating out water having glass fibers, connected with islands of thermoplastic polymers by melting the polymers, and further coated with a hydrophobic agent such as a phenolic resin. Reick, U.S. Pat. No. 3,976,572, discloses a fuel chamber separated by a selectively permeable filter that only allows fuel to pass into the chamber leading to an engine; the filter is made of a polyester nonwoven coated and bonded to hydrophobically treated silica particles. Lewis, U.S. Pat. No. 4,372,847, describes a fuel filter assembly that includes a hydrophobically treated filter material for coalescing water while allowing fuel to pass through without coalescing. Fischer et al., U.S. Pat. Nos. 4,512,882 and 4,522,712, teach a hydrophobic coating on a paper-type filter element to separate water from fuel. Cole et al., U.S. Pat. No. 4,787,949 disclose hydrophilic superabsorbent materials covered by a nonwoven for sacrificial removal of water from fuel. Taylor, U.S. Pat. Nos. 4,790,941, 4,814,087, and 4,850,498 teaches the use of tangential flow filtration of fuel employing a plurality of microporous hydrophobic hollow fiber membranes to separate fuel from e.g. seawater. Davis, U.S. Pat. No. 5,507,942 discloses a vertical fuel filter that can have a hydrophobic coating on a paper nonwoven to prevent the passage of water into the engine. Kheyfets, U.S. Pat. No. 5,904,956 teaches a filter element for separating water from fuel that includes fibers having a first treatment of a cationic surfactant in an aromatic hydrocarbon solvent, followed by a silane surface treatment. Goodrich, U.S. Pat. Nos. 5,916,442, 6,203,698, and 6,540,926 discloses an annularly spaced, vertical filter assembly that separates water from fuel by gravity as the fuel flows upward through the narrow annular channel. The filtering means can be a hydrophobic media that prohibits the passage of water through the media. Schroll, U.S. Pat. No. 5,989,318 teaches a filter assembly wherein a hydrophobic filter is disposed between the fuel intake and the engine to prevent water from passing into the engine. Hagerthy, U.S. Pat. No. 5,993,675 discloses use of a plurality of petroleum sorbent filter elements, constructed from multiple adjacent microfiber layers that allow fuel but not water to pass through. Surface layers contacting the fuel source can be heated to coalesce and bond the fibers.
Berger, U.S. Pat. Nos. 6,103,181, 6,576,034, 6,616,723, and 7,192,550 discloses a nonwoven web of fibers having more than one fiber type or more than one component per fiber (e.g. bicomponent fibers); several applications for such fibers are disclosed, such as separation of water from aviation fuel. Fluorocarbon and chlorocarbon polymers are provided as possible components of filter fibers for dust collection from e.g. air. Condran et al., U.S. Pat. No. 6,371,087 teach a filter coated with phenolic resin containing silicone to separate water from fuel in a locomotive fuel application. Li et al., U.S. Pat. No. 6,422,396 teach a hydrophobic filter medium having a coalescing layer of fine fiber and a negative porosity gradient to promote enlargement of trapped liquid droplets. Whitehead et al., U.S. Pat. No. 6,526,741 disclose a fuel filter media having a prefilter and a hydrophobic water coalescing media, wherein the prefilter protects the coalescing filter from particulate clogging. Gorman et al., U.S. Pat. No. 6,764,598 disclose a hydrophobic depth filter and a PTFE cross-flow membrane filter that separates and coalesces surfactant and water contaminants from fuel. Yu et al., U.S. Patent Pub. No. 2004/0222156 teach an apparatus for filtration of surfactant and water from fuel, having a hydrophobic water coagulation depth filter cartridge and a hydrophobic water separation cartridge. Klein et al., U.S. Pat. Pub. No. 2006/0006109 disclose a serially connected particle and coalescing filter, wherein the coalescing filter is a hydrophobic nonwoven that separates water from fuel without exhibiting any particle filtration properties.
Fluorinated polymers have occupied a predominant place among the specialty polymers, as their unique combination of properties has made them the materials of choice in a wide number of areas. Fluoropolymers' outstanding chemical inertness, low surface energy, along with remarkable mechanical and heat tolerance properties, find them applications that span a wide range of technological areas. Other properties of importance, such as low flammability and low temperature flexibility, are also typical of fluoropolymers. The ubiquitous example of the chemical inertness and temperature resistance is the fully fluorinated PTFE, which properties in that regard are unmatched, and which since its insertion in the market has constituted an important component in a large number of durables and consumables.
The prior art contains disclosures of fluoropolymers used in making nonwoven layers. Fletcher et al., U.S. Pat. Nos. 3,956,233 and 4,094,943, teach conventional filament comprising block urethanes combined with fluoropolymer materials for increasing flame retardancy in nonfluorinated polymers used to make fabrics. Pall et al., U.S. Pat. Nos. 4,594,202 and 4,726,901, teach melt blown processed fluoropolymer fiber materials. Karbachsch et al., U.S. Pat. No. 4,983,288, teach the use of PVDF or PTFE for use in the membrane type filter materials. McGregor et al., U.S. Pat. No. 5,264,276, teach a laminate structure that can be made of a variety of porous fluoropolymers. A layer comprising a non-porous layer and a fibrous layer is used. The fiber layer can comprise Viton® type elastomer materials. Walla et al., U.S. Pat. No. 5,908,528, teach a protective reinforced fibrous fluoroelastomer layer that can contain a nanofiber. Cistone et al., U.S. Publication No. 2002/0155289 A1, teach melt processable woven, non-woven and knitted fluoropolymer structures for use in filters and support media.
None of the prior art references are drawn to electrospinning of fluoropolymer solutions to provide a fine fiber layer. Electrospinning has emerged as a convenient technique for the fabrication of micro and nanoscale objects, and it is particularly useful in the fabrication of fibrous matrices. In the fabrication of fibers electrospinning has an advantage over traditional technologies, such as melt-spinning and melt-blowing, of operating at room temperature. Another significant advantage, and one that is unique to electrospinning, is its ability of producing fibers in the submicron scale.
Electrospinning is typically carried out from a polymer solution, wherein the solvent is the major component. A hypodermic needle, nozzle, capillary or movable emitter provides liquid solutions of the polymer that are then attracted to a collection zone by a high voltage electrostatic field. As the materials are pulled from the emitter and accelerate through the electrostatic zone, the fiber becomes very thin and can be formed in a fiber structure by solvent evaporation. Such techniques are described by, for example, Reneker, D. et al., “Nanometer diameter fibres of polymer, produced by electrospinning,” Nanotechnology, vol. 7, pp. 216-223 (1996); and Baumgarten, P., “Electrostatic Spinning of Acrylic Microfibers”, J. Colloid and Interface Sci. vol. 36, No. 1, 9 pages (May 1971).
Of the fluorinated homopolymers, only a selected number of them are soluble in organic solvents at room temperature. One of the few known solvents for fluorinated homopolymers are fluorinated solvents. For example, Tuminello et al., U.S. Pat. Nos. 5,328,946 and 5,683,557 describe perfluorinated cycloalkane solvents for dissolving high melting polymers containing tetrafluoroethylene. These solvents are expensive and can be detrimental to the environment. Other fluorinated homopolymers are soluble in organic solvents that are undesirable to use for electrospinning. For example, poly(vinylidene fluoride), a fluorinated homopolymer, is only soluble in highly polar aprotic solvents such as DMF, DMAC, and the like.
Because of their very high hydrophobicity, fluorinated materials have potential as barrier filter materials to remove water from contaminated fuel, even where the water is emulsified and therefore not separable by conventional means. Fine fiber filters made by electrospinning are excellent candidates for such a filter. However, fluorinated polymers are generally poor candidates for electrospinning due to solubility issues. If the material under consideration for electrospinning requires the use of hazardous solvents, this can provide either major solvent management challenges or make the process prohibited altogether. Additionally, the choice of solvent in electrospinning has a significant impact on the final morphological features of the matrix, as well as the stability and productivity of the process. It is therefore desirable to employ the safest solvents, while ensuring stability and continuity of the process, and providing the expected quality and performance of the resulting fine fiber webs.
Thus, there is a need in the industry for a coalescing filter material made by electrospinning fluoropolymers having solubility in relatively benign solvents. There is a further need in the industry for a filter construction that can effectively remove emulsified water particles from fuel materials.